Optical Network Unit Wavelength Tuning

ABSTRACT

A method of establishing communication between an optical line terminal and an optical network unit within an optical access network includes receiving a signal indication from an optical transceiver of an optical line terminal. The signal indication includes: (i) a loss-of-signal indication indicating non-receipt of an upstream optical signal from the optical network unit; or (ii) a signal-received indication indicating receipt of the upstream optical signal from the optical network unit. The method includes determining whether the signal indication includes the loss-of-signal indication. When the signal indication includes the loss-of-signal indication, the method includes instructing the optical transceiver to cease signal transmission from the optical transceiver to the optical network unit. Moreover, when the signal indication includes the signal-received indication, the method includes instructing the optical transceiver to transmit a downstream optical signal from the optical transceiver to the optical network unit.

TECHNICAL FIELD

This disclosure relates to tuning wavelengths in wavelength divisionmultiplexed (WDM) passive optical networks (PONs).

BACKGROUND

Fiber optic communication is an emerging method of transmittinginformation from a source (transmitter) to a destination (receiver)using optical fibers as the communication channel. WDM-PON is an opticaltechnology for access and backhaul networks. WDM-PON uses multipledifferent wavelengths over a physical point-to-multipoint fiberinfrastructure that contains passive optical components. The use ofdifferent wavelengths allows for traffic separation within the samephysical fiber. The result is a network that provides logicalpoint-to-point connections over a physical point-to-multipoint networktopology. WDM-PON allows operators to deliver high bandwidth to multipleendpoints over long distances. A PON generally includes an optical lineterminal located at a service provider central office (e.g., a hub) anda number of optical network units or optical network terminals, near endusers. The optical line terminal may include many optical transceiverspaired with the optical network units and assigned a correspondingchannel. While the optical transceivers of the optical line terminal maybe pre-tuned to their assigned channels for optical communications tothe optical network units, the optical network units may need to betuned to their corresponding assigned channels as well.

SUMMARY

The present disclosure provides systems and methods for establishingcommunication between an optical line terminal and an optical networkunit within an optical access network. According to one aspect, a methodincludes receiving, at data processing hardware, a signal indicationfrom an optical transceiver of an optical line terminal. The signalindication includes: (i) a loss-of-signal indication indicatingnon-receipt of an upstream optical signal from the optical network unit;or (ii) a signal-received indication indicating receipt of the upstreamoptical signal from the optical network unit. The method includesdetermining, by the data processing hardware, whether the signalindication includes the loss-of-signal indication. When the signalindication includes the loss-of-signal indication, the method includesinstructing, by the data processing hardware, the optical transceiver tocease signal transmission from the optical transceiver to the opticalnetwork unit. Moreover, when the signal indication includes thesignal-received indication, the method includes instructing, by the dataprocessing hardware, the optical transceiver to transmit a downstreamoptical signal from the optical transceiver to the optical network unit.

Implementations of the disclosure may include one or more of thefollowing optional features. In some implementations, the opticalnetwork unit is configured to recursively transmit the upstream opticalsignal at different wavelengths until receipt of the downstream opticalsignal. In some examples, the optical network unit is configured torecursively transmit the upstream optical signal at sequentiallydifferent wavelengths within a free spectral range. Additionally oralternatively, the optical network unit may be configured to delaytransmission of a subsequent upstream optical signal after a priorupstream optical signal by a threshold period of time. The upstreamoptical signal and the downstream optical signal may have the same ordifferent wavelengths.

Another aspect of the disclosure provides an optical line terminal thatincludes an optical transceiver, data processing hardware incommunication with the optical transceiver, and memory hardware incommunication with the data processing hardware. The memory hardwarestores instructions that when executed on the data processing hardwarecause the data processing hardware to perform operations. The operationsinclude receiving a signal indication from the optical transceiver. Thesignal indication includes: (i) a loss-of-signal indication indicatingnon-receipt of an upstream optical signal from the optical network unit;or (ii) a signal-received indication indicating receipt of the upstreamoptical signal from the optical network unit. The operations includedetermining whether the signal indication includes the loss-of-signalindication. When the signal indication includes the loss-of-signalindication, the operations include instructing the optical transceiverto cease signal transmission from the optical transceiver to the opticalnetwork unit. Moreover, when the signal indication comprises thesignal-received indication, the operations include instructing theoptical transceiver to transmit a downstream optical signal from theoptical transceiver to the optical network unit.

This aspect may include one or more of the following optional features.In some implementations, the optical network unit is configured torecursively transmit the upstream optical signal at differentwavelengths until receipt of the downstream optical signal. Additionallyor alternatively, the optical network unit may be configured to delaytransmission of a subsequent upstream optical signal after a priorupstream optical signal by a threshold period of time. In some examples,the optical network unit is configured to recursively transmit theupstream optical signal at sequentially different wavelengths within afree spectral range. The upstream optical signal and the downstreamoptical signal may have the same or different wavelengths.

Another aspect of the disclosure provides a method of establishingcommunication between an optical line terminal and an optical networkunit within an optical access network. The method includes instructing,by data processing hardware, an optical transceiver to recursivelytransmit an upstream optical signal at different wavelengths to anoptical line terminal. For each transmitted upstream optical signal, themethod includes determining, by the data processing hardware, whetherthe optical transceiver received a downstream optical signal from theoptical line terminal within a threshold period of time. When theoptical transceiver received the downstream optical signal from theoptical line terminal, the method includes instructing, by dataprocessing hardware, the optical transceiver to cease changingwavelengths of the upstream optical signal.

This aspect may include one or more of the following optional features.In some implementations, instructing the optical transceiver torecursively transmit the upstream optical signal at differentwavelengths includes delaying transmission of a subsequent upstreamoptical signal after a prior upstream optical signal by a thresholddelay. Additionally or alternatively, instructing the opticaltransceiver to recursively transmit the upstream optical signal atdifferent wavelengths includes transmitting the upstream optical signalat sequentially different wavelengths within a free spectral range. Theupstream optical signal and the downstream optical signal may have thesame or different wavelengths. In some examples, the method includesstoring in memory hardware at least one of: (i) the wavelength of theupstream optical signal when the optical transceiver received thedownstream optical signal from the optical line terminal; or (ii) thewavelength of the downstream optical signal.

Another aspect of the disclosure provides an optical network unit thatincludes an optical transceiver, data processing hardware incommunication with the optical transceiver, and memory hardware incommunication with the data processing hardware. The memory hardwarestores instructions that when executed on the data processing hardwarecause the data processing hardware to perform operations. The operationsinclude instructing an optical transceiver to recursively transmit anupstream optical signal at different wavelengths to an optical lineterminal. For each transmitted upstream optical signal, the operationsinclude determining, whether the optical transceiver received adownstream optical signal from the optical line terminal within athreshold period of time. When the optical transceiver received thedownstream optical signal from the optical line terminal, the operationsinclude instructing the optical transceiver to cease changingwavelengths of the upstream optical signal.

This aspect may include one or more of the following optional features.In some implementations, instructing the optical transceiver torecursively transmit the upstream optical signal at differentwavelengths includes delaying transmission of a subsequent upstreamoptical signal after a prior upstream optical signal by a thresholddelay. Additionally or alternatively, instructing the opticaltransceiver to recursively transmit the upstream optical signal atdifferent wavelengths includes transmitting the upstream optical signalat sequentially different wavelengths within a free spectral range. Theupstream optical signal and the downstream optical signal may have thesame or different wavelengths. In some examples, the operations includestoring in memory hardware at least one of: (i) the wavelength of theupstream optical signal when the optical transceiver received thedownstream optical signal from the optical line terminal; or (ii) thewavelength of the downstream optical signal.

Another aspect of the disclosure provides a method for establishingcommunication between an optical line terminal and an optical networkunit within an optical access network. The method includes receiving, atdata processing hardware, a signal indication from an opticaltransceiver of an optical line terminal. The signal indication includesa loss-of-signal indication indicating non-receipt of an upstream signalfrom an optical network unit or a signal-received indication indicatingreceipt of the upstream optical signal from the optical network unit.The method also includes determining, by the data processing hardware,whether the signal indication includes the loss-of-signal indication.When the signal indication includes the loss-of-signal indication, themethod includes instructing, by the data processing hardware, enablementand disablement of the optical transceiver to transmit a downstreamoptical signal to the optical network unit at a threshold bit rate,resulting in a modulated downstream optical signal including encodedchannel information.

This aspect may include one or more of the following optional features.In some implementations, when the signal indication includes thesignal-received indication, the method may include instructing, by thedata processing hardware, the optical transceiver to continuouslytransmit the downstream optical signal from the transceiver to theoptical network unit according to the channel information. The encodedchannel information may include an x-bit long delimiter segment and ay-bit long channel segment.

Yet another aspect of the disclosure provides an optical line terminalincluding an optical transceiver, data processing hardware incommunication with the optical transceiver, and memory hardware incommunication with the data processing hardware. The memory hardwarestores instructions that when executed on the data processing hardwarecause the data processing hardware to perform operations. The operationsinclude receiving a signal indication from the optical transceiver anddetermining whether the signal indication includes the loss-of-signalindication. The signal indication includes a loss-of-signal indicationindicating non-receipt of an upstream optical signal from an opticalnetwork unit or a signal-received indication indicating receipt of theupstream optical signal from the optical network unit. When the signalindication includes the loss-of-signal indication, the method includesinstructing enablement and disablement of the optical transceiver totransmit a downstream optical signal to the optical network unit at athreshold rate, resulting in a modulated downstream optical signalincluding encoded channel information.

This aspect may include one or more of the following optional features.In some implementations, the operations further include, when the signalindication includes the signal-received indication, instructing theoptical transceiver to continuously transmit the downstream opticalsignal from the transceiver to the optical network unit according to thechannel information. The encoded channel information may include anx-bit long delimiter segment and a y-bit long channel segment.

Yet another aspect of the disclosure provides a method for establishingcommunication between an optical line terminal and an optical networkunit within an optical access network. The method includes receiving, atdata processing hardware, a signal indication from an opticaltransceiver of an optical network unit and determining, by the dataprocessing hardware, whether the signal indication includes theloss-of-signal indication. The signal indication includes aloss-of-signal indication indicating non-receipt of a downstream opticalsignal from an optical line terminal or a signal-received indicationindicating receipt of the downstream optical signal from the opticalline terminal. When the signal indication includes the loss-of-signalindication, the method includes receiving, at the data processinghardware, a modulated downstream optical signal at a threshold bit rate,determining, by the data processing hardware, encoded channelinformation of the modulated downstream optical signal based on thethreshold bit rate, and tuning, by the data processing hardware, theoptical transceiver according the encoded channel information.

This aspect may include one or more of the following optional features.In some implementations, the encoded channel information may include anx-bit long delimiter segment and a y-bit long channel segment.Determining the encoded channel information may include rotating thesegments of the encoded channel information, identifying the x-bit longdelimiter segment, and identifying the y-bit long channel segment aschannel information.

Yet another aspect of the disclosure provides an optical network unitincluding an optical transceiver, data processing hardware incommunication with the optical transceiver, and memory hardware incommunication with the data processing hardware. The memory hardwarestores instructions that when executed on the data processing hardwarecause the data processing hardware to perform operations. The operationsinclude receiving a signal indication from the optical transceiver anddetermining whether the signal indication comprises the loss-of-signalindication. The signal indication includes a loss-of-signal indicationindicating non-receipt of a downstream optical signal from an opticalline terminal or a signal-received indication indicating receipt of thedownstream optical signal from the optical line terminal. When thesignal indication includes the loss-of-signal indication, the operationsinclude receiving a modulated downstream optical signal at a thresholdbit rate, determining encoded channel information of the modulateddownstream optical signal based on the threshold bit rate, and tuningthe optical transceiver according the encoded channel information.

This aspect may include one or more of the following optional features.In some implementations, the encoded channel information includes anx-bit long delimiter segment and a y-bit long channel segment.Determining the encoded channel information may include rotating thesegments of the encoded channel information, identifying the x-bit longdelimiter segment, and identifying the y-bit long channel segment aschannel information.

The details of one or more implementations of the disclosure are setforth in the accompanying drawings and the description below. Otheraspects, features, and advantages will be apparent from the descriptionand drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an example communication system.

FIGS. 2A and 2B are schematic views of example arrayed waveguidegratings.

FIGS. 3-6 are schematic views of example arrangements of operations formethods of establishing communication between an optical line terminaland an optical network unit within an optical access network.

FIG. 7 is schematic view of an example computing device that may be usedto implement the systems and methods described in this document.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, a communication system 100 delivers communicationsignals 102 (e.g., optical signals) through communication links 110, 110a-n (e.g., optical fibers or line-of-sight free space opticalcommunications) between an optical line terminal (OLT) 120 housed in acentral office (CO) 130 and optical network units (ONUs) 140, 140 a-n(e.g., a bidirectional optical transceiver) associated with users 150,150 a-n (also referred to as customers or subscribers). The ONUs 140,140 a-n are typically located at premises 152, 152 a-n of the users 150,150 a-n.

Customer premises equipment (CPE) is any terminal and associatedequipment located at the premises 152 of the user 150 and connected to acarrier telecommunication channel C at a demarcation point (“demarc”).In the examples shown, the ONU 140 is a CPE. The demarc is a pointestablished in a house, building, or complex to separate customerequipment from service provider equipment. CPE generally refers todevices such as telephones, routers, switches, residential gateways(RG), set-top boxes, fixed mobile convergence products, home networkingadapters, or Internet access gateways that enable the user 150 to accessservices of a communications service provider and distribute them aroundthe premises 152 of the user 150 via a local area network (LAN).

In some implementations, the optical communication system 100 implementsan optical access network 105, such as a passive optical network (PON)105. In some implementations, the optical communication system 100implements a point-to-point (pt-2-pt) PON having direct connections,such as optical Ethernets, where a home-run optical link 110 (e.g.,fiber) extends all the way back to an OLT 120 at the CO 130 and eachcustomer 30, 30 a-n is terminated by a separate OLT 120 a-n, as opposedto the shared OLT 120.

The CO 130 includes at least one OLT 120 connecting the optical accessnetwork 105 to an Internet Protocol (IP), Asynchronous Transfer Mode(ATM), or Synchronous Optical Networking (SONET) backbone, for example.Therefore, each OLT 120 is an endpoint of the PON 105 and convertsbetween electrical signals used by service provider equipment and opticsignals 102 used by the PON 105. The OLT 120 sends the optical signal102 through a feeder fiber 110 to the remote node 170, whichdemultiplexes the optical signal 102 and distributes the demulitplexedoptical signals 102 to multiple users 150, 150 a-n. The multiplexer 160for multiplexing/demultiplexing splitters may be an arrayed wavelengthgrating (AWG), which is a passive optical device. In some examples, eachCO 130 includes multiple OLTs 120, 120 a-n, and each OLT 120 isconfigured to service a group of users 150. In addition, each OLT 120may be configured to provide signals in different services, e.g., oneOLT 120 may provide services in 1G-PON, while another OLT 120 providesservices in 10G-PON.

As shown in FIG. 1, the CO 130 multiplexes signals received from severalsources, such as video media distribution 132, Internet data 134, andvoice data 136, and multiplexes the received signals into onemultiplexed signal 102 before sending the multiplexed optical signal 102to the remote node 170 through the feeder fiber 110. The multiplexingmay be performed by the OLT 120 or a broadband network gateway (BNG)positioned at the CO 130. Typically, services aretime-division-multiplexed (TDM) on the packet layer. When the CO 130includes more than one OLT 120, the signals 102 of multiple OLT 120, 120a-n can be multiplexed to form a wavelength division multiplexed (WDM))signal for delivery to the remote node 170. Multiplexing combinesseveral input signals and outputs a combined signal.

A Wavelength Division Multiplexing (WDM) PON 105 is a fiber opticalnetwork architecture for access and mobile fronthaul/backhaul networks.The WDM-PON 105 uses multiple wavelengths λ to implementpoint-to-multi-point communications. The OLT 120 serves multiplewavelengths through one fiber 110 to the multiplexer 160 at the remotenode 170, which multiplexes/demultiplexes signal between the OLT 120 anda plurality of ONUs 140, 140 a-n.

FIGS. 2A and 2B illustrate an exemplary arrayed waveguide grating 200(AWG), which may be used as a multiplexer 160. An AWG 200 may be used todemultiplex an optical signal in a WDM system. AWGs 200 can multiplex alarge number of wavelengths λ into one optical fiber, thus increasingthe transmission capacity of optical networks. AWGs 200 can thereforemultiplex channels of several wavelengths λ onto a single optical fiberat a transmission end, and reciprocally they can also demultiplexdifferent wavelength channels at the receiving end of an optical accessnetwork 105. An AWG 200 is a passive planar light wave circuit devicetypically used as a wavelength multiplexer and/or demultiplexer. N×NAWGs 200 also have wavelength routing capabilities. If a system has Nequally-spaced wavelengths λ_(n), an N×N AWG 200 can be designed with anegress port spacing matching the wavelength spacing. The N×N AWG 200routes differing wavelengths λ, λ_(1-n) at an ingress port 210, 210 a-nto different egress ports 220, 220 a-n such that all N wavelengthsλ_(1-n) are mapped to all N egress ports 220 a-n sequentially. Therouting of the same N wavelengths λ_(1-n) at two consecutive ingressports 210 have the wavelength mapping shifted by one egress side.

The AWG 200 is cyclic in nature. The wavelength multiplexing anddemultiplexing property of the AWG 200 repeats over periods ofwavelengths called free spectral range (FSR). Multiple wavelengths,separated by the free spectral range (FSR), are passed down each port220. Therefore, by utilizing multiple FSR cycles, different tieredservices may coexist on the same fiber plant 20, 22.

Referring again to FIG. 1, the OLT 120 includes multiple opticaltransceivers 122, 122 a-n. Each optical transceiver 122 transmitssignals at one fixed wavelength λ_(D) (referred to as a downstreamwavelength) and receives optical signals 102 at one fixed wavelengthλ_(U) (referred to as an upstream wavelength). The downstream andupstream wavelengths λ_(D), λ_(U) may be the same or different.Moreover, a channel C may define a pair of downstream and upstreamwavelengths λ_(D), λ_(U), and each optical transceiver 122, 122-n of acorresponding OLT 120 may be assign a unique channel C_(a-n).

The OLT 120 multiplexes/demultiplexes the channels C, C_(a-n) of itsoptical transceivers 122, 122 a-n for communication of an optical signal102 through the feeder fiber 110. Whereas, the multiplexer 160 at theremote node 170 multiplexes/demultiplexes optical signals 102, 104,104-n between the OLT 120 and a plurality of ONUs 140, 140 a-n. Forexample, for downstream communications, the multiplexer 160demultiplexes the optical signal 102 from the OLT 120 into ONU opticalsignals 104, 104-n for each corresponding ONU 140, 140 a-n. For upstreamcommunications, the multiplexer 160 multiplexes ONU optical signals 104,104-n from each corresponding ONU 140, 140 a-n into the optical signal102 for delivery to the OLT 120. To make the transmission successful,the optical transceivers 122, 122 a-n of the OLT 120 match with the ONUs140, 140-n one-by-one. In other words, the downstream and upstreamwavelengths λ_(D), λ_(U) (i.e., the channel C) of a given ONU opticalsignal 104, 104-n matches the downstream and upstream wavelengths λ_(D),λ_(U) (i.e., the channel C) of a corresponding optical transceiver 122.In some examples, the ONU 140 includes a photodetector that converts theoptical wave to an electric form. The electrical signal may be furtherde-multiplexed down to subcomponents (e.g., data over a network, soundwaves converted into currents using microphones and back to its originalphysical form using speakers, converting images converted into currentsusing video cameras and converting back to its physical form using atelevision).

In some implementations, each ONU 140, 140 a-n includes a correspondingtunable ONU transceiver 142, 142 a-n (e.g., that includes a laser orlight emitting diode) that can tune to any or partial wavelength λ usedby a corresponding OLT 120 at a receiving end. The ONU transceiver 142,142 a-n facilitates easier in-field installation and better inventorymanagement than a fixed wavelength transmitter. When installing an ONU140, it is desirable to have the ONU 140 automatically tune to awavelength λ that establishes a communication link between thecorresponding OLT 120 and the ONU 140.

In some implementations, the automatic wavelength tuning can beimplemented above a physical layer. For example, if the OLT 120 andcorresponding ONUs 140, 140 a-n communicate by Ethernet protocol, theOLT 120 can instruct each transceiver 122, 122 a-n to send Ethernetbroadcast packets that includes channel information. After receiving thepacket, each ONU 140 can: (i) decode the channel information; (ii)self-tune the corresponding ONU transceiver 142 to the downstream andupstream wavelengths λ_(D), λ_(U) of the channel C assigned to the ONU140 based on the channel information; and (iii) establish bi-directionalcommunication with the corresponding OLT 120. Moreover, the OLT 120 canuse other high-level protocols, such as Simple Network ManagementProtocol (SNMP), Technical Report 069 (TR069), etc. to send the channelinformation to the ONUs 140, 140 a-n. A disadvantage of this solution,however, is that the implementation is tied to the higher-layerprotocols. As a result, any protocol change means re-implementing thetuning solution, which may prompt an equipment upgrade or replacement.

In alternative implementations, the OLT 120 implements a tone-basedsolution that works at the physical layer. In such implementations, theOLT 120 modulates the optical transceivers 122, 122-n with low-frequencytones. The ONUs 140, 140 a-n detect the tone frequency and accompanyingtone frequency information mapped to the channels C, C_(a-n). Thisimplementation, however, need special circuit to detect the tone andincreases the cost of the ONUs 140, 140 a-n.

In some implementations, the automatic wavelength tuning can beimplemented in the physical layer. The optical transceivers 122 have thecapacity to detect whether a received optical signal 102 exists or notand indicate a Loss-Of-signal (LOS) state. LOS is an indicator on anetworking device, such as the OLT 120, indicating whether a signal orconnection has been dropped or terminated. When LOS is asserted, itmeans the optical transceiver 122 is not receiving any optical signal102. The optical transceivers 122 can also enable or disable signaltransmission. This control is referred to as Tx Enable and Tx Disable.Firmware of each optical transceiver 122 controls the Tx Enable and TxDisable functionality and reads the LOS state. Each optical transceiver122, 142 may include data processing hardware 124, 144 (e.g., circuitry,field programmable gate arrays (FPGAs, etc.) and memory hardware 126,146 in communication with the data processing hardware 124, 144. Thememory hardware 126, 146 may store instructions (e.g., via firmware)that when executed on the data processing hardware 124, 144 cause thedata processing hardware 124, 144 to perform operations for auto-tuningthe optical transceiver 122, 142.

With continued reference to FIG. 1, in some implementations, an OLT 120and a corresponding ONU 140 interact with each other to implementauto-tuning of the ONU 140. The OLT 120 may instruct its opticaltransceiver 122 designated for the ONU 140 to set the Tx Disable todisable transmission of any downstream optical signal 104 d to the ONU140 when the optical transceiver 122 asserts the LOS state. When theoptical transceiver 142 of the ONU 140 asserts its LOS state, however,the ONU 140 instructs its optical transceiver 142 to set the Tx Enableand actively switch channels C while transmitting an upstream opticalsignal 104 u. Once the ONU 140 switches to the channel C assigned to theONU 140 (e.g., the channel C having the downstream and upstreamwavelengths λ_(D), λ_(U) assigned to the ONU 140), the opticaltransceiver 122 of the OLT 120 receives the upstream optical signal 104u and de-asserts its corresponding LOS state. When the OLT 120determines that its optical transceiver 122 is no longer asserting itsLOS state, the OLT 120 instructs its optical transceiver 122 to set TxEnable and transmit a downstream optical signal 104 d to the ONU 140.The ONU 140 accepts the downstream optical signal 104 d and stopschannel hunting. At each wavelength λ, the ONU 140 waits a period oftime between transmissions long enough for the OLT transceiver 142 torespond to each transmission of the upstream optical signal 104 u.

In some implementations, the ONU 140 actively switches channel when theoptical transceiver 142 asserts the LOS state is asserted, until theoptical transceiver 142 accepts the downstream optical signal 104 d fromthe OLT 120. A channel switch time of the ONU 140 while channel huntingmay be greater than or equal to a sum of T1, T2, T3, T4, T5, and T6,where:

-   -   {circle around (1)} T1 is a wavelength tuning time until        wavelength stability (e.g., about 1 s);    -   {circle around (2)} T2 is RX_LOS time of the ONU 140 (e.g., less        than 100 μs);    -   {circle around (3)} T3 is a response time of a microcontroller        144 of the ONU 140 (e.g., less than 25 ms);    -   {circle around (4)} T4 is a RX_LOS time of the OLT 120 (e.g.,        less than 100 μs);    -   {circle around (5)}T5 is a TX_Enable time of the OLT 120 (e.g.,        less than 1 ms); and

-   {circle around (6)} T6 is a light transmission time from the ONU 140    to the OLT 120 and then from the OLT 120 back to the ONU 140 (e.g.,    less than 1 ms).    An example channel switch time may be about two seconds.

FIG. 3 provides an example arrangement of operations for a method 300 ofestablishing communication between an OLT 120 and an ONU 140 within anoptical access network 105. At block 302, the method 300 includesreceiving, at data processing hardware 124, a signal indication 148 froman optical transceiver 122 of an OLT 120. The signal indication 128includes: (i) a loss-of-signal (LOS) indication 128 a indicatingnon-receipt of an upstream optical signal 104 u from the ONU 140; or(ii) a signal-received indication 128 b indicating receipt of theupstream optical signal 104 u from the ONU 140. At block 304, the method300 includes determining, by the data processing hardware 124, whetherthe signal indication 128 includes the loss-of-signal indication 128 a.When the signal indication 128 includes the loss-of-signal indication128 a, at block 306, the method 300 includes instructing, by the dataprocessing hardware 124, the optical transceiver 122 of the OLT 120 tocease signal transmission to the ONU 140 (e.g., via Tx Disable). Atblock 308, when the signal indication 128 includes the signal-receivedindication 128 b, the method 300 includes instructing, by the dataprocessing hardware 124, the optical transceiver 122 to transmit adownstream optical signal 104 d to the ONU 140 (e.g., via Tx Enable).

In some implementations, the ONU 140 is configured to recursivelytransmit the upstream optical signal 104 u at different wavelengths λuntil receipt of the downstream optical signal 104 d. In some examples,the ONU 140 is configured to recursively transmit the upstream opticalsignal 104 u at sequentially different wavelengths λ within a freespectral range. Additionally or alternatively, the ONU 140 may beconfigured to delay transmission of a subsequent upstream optical signal104 u after a prior upstream optical signal 104 u by a threshold periodof time. For example, at each wavelength λ, the ONU 140 waits betweentransmissions long enough for the optical transceiver 122 of the OLT 120to respond to the transmission. The upstream optical signal 104 u andthe downstream optical signal 104 d may have the same or differentwavelengths λ.

FIG. 4 provides an example arrangement of operations for a method 400 ofestablishing communication between an OLT 120 and an ONU 140 within anoptical access network 105. At block 402, the method 400 includesinstructing, by data processing hardware 144, an optical transceiver 142of an ONU 140 to recursively transmit an upstream optical signal 104 uat different wavelengths λ to an OLT 120 (e.g., via ONU channelhunting). At block 404, for each transmitted upstream optical signal 104u, the method 400 includes determining, by the data processing hardware144, whether the optical transceiver 142 received a downstream opticalsignal 104 d from the OLT 120 within a threshold period of time. Atblock 406, when the optical transceiver 142 receives the downstreamoptical signal 104 d from the OLT 120, the method 400 includesinstructing, by data processing hardware 144, the optical transceiver142 to cease changing wavelengths λ of the upstream optical signal 104u. In other words, the ONU 140 accepts the light and stops channelhunting.

In some implementations, instructing the optical transceiver 142 of theONU 140 to recursively transmit the upstream optical signal 104 u atdifferent wavelengths λ includes delaying transmission of a subsequentupstream optical signal 104 u after a prior upstream optical signal 104u by a threshold delay. For example, at each wavelength λ, the ONU 140waits between transmissions long enough for the optical transceiver 122of the OLT 120 to respond to the transmission. Additionally oralternatively, instructing the optical transceiver 142 of the ONU 140 torecursively transmit the upstream optical signal 104 u at differentwavelengths λ includes transmitting the upstream optical signal 104 u atsequentially different wavelengths λ within a free spectral range. Theupstream optical signal 104 u and the downstream optical signal 104 dmay have the same or different wavelengths λ. In some examples, themethod 400 includes storing in memory hardware 146 at least one of: (i)the wavelength λ of the upstream optical signal 104 u when the opticaltransceiver 142 of the ONU 140 received the downstream optical signal104 d from the OLT 120; or (ii) the wavelength λ of the downstreamoptical signal 104 d.

Referring again to FIG. 1, in some implementations, an OLT 120 and acorresponding ONU 140 interact with each other to implement auto-tuningof the ONU 140 via encoded channel information. The OLT 120 controls theTx Enable/Tx Disable of the optical transceiver 122 corresponding to theONU 140 (e.g., via firmware) to modulate transmission of a downstreamoptical signal 104 d to the ONU 140. The downstream optical signal 104 dincludes encoded channel information for the channel C assigned to theONU 140. In some examples, the encoded channel information includes anx-bit long delimiter segment and a y-bit long channel segment.

Encoding and Modulation are two techniques used to provide the means ofmapping information or data into different waveforms such that thereceiver (with the help of an appropriate demodulator and decoder) canrecover the information in a reliable manner. Encoding is the process bywhich the data is converted into digital format for efficienttransmission or storage. Modulation is the process of convertinginformation (signals or data) to an electronic or optical carrier, sothat it can be transmitted to comparatively large distance withoutgetting affected by noise or unwanted signals.

The OLT 120 instructs its optical transceiver 122 to transmit thedownstream optical signal 104 d at a slow speed (e.g., 10 bits/s orslower) when the optical transceiver 122 of the OLT 120 asserts the LOSstate. The optical transceiver 122 can achieve this by controlling theTx Enable control through firmware.

In light of the slow transmission speed, the ONU 140 can use a firmwareloop there inside to receive the channel information by monitoring theONU LOS indictor at the same bit rate. After receiving x+y bits, the ONU140 can rotate the received bits (e.g., via firmware) until thedelimiter is identified as the first part of the transmission. The ONU140 uses the second part of the transmission as channel information andtunes the ONU optical transceiver 142 accordingly. After the ONU 140self-tunes to the correct channel C (e.g., having the correct downstreamand upstream wavelengths λ_(D), λ_(U) assigned to the ONU 140), the ONU140 sets the Tx Enable and transmits an upstream optical signal 104 u tothe OLT 120. The optical transceiver 122 of the OLT 120 accepts upstreamoptical signal 104 u at the correct channel C, thereby causing the OLT120 to de-assert the LOS state. Once out of the LOS state, the OLT 120stops the channel signaling and commences normal operation.

FIG. 5 provides an example arrangement of operations for a method 500 ofestablishing communication between an OLT 120 and an ONU 140 within anoptical access network 105. At block 502, the method 500 includesreceiving, at data processing hardware 124, a signal indication 128 froman optical transceiver 122 of an optical line terminal 120. The signalindication 128 includes a loss-of-signal indication 128 a indicatingnon-receipt of an upstream signal 104 u from an optical network unit 140or a signal-received indication 128 n indicating receipt of the upstreamoptical signal 104 u from the optical network unit 140. At block 504,the method 500 also includes determining, by the data processinghardware, whether the signal indication 128 includes the loss-of-signalindication 128 a. At block 506, when the signal indication 128 includesthe loss-of-signal indication 128 a, the method 500 includesinstructing, by the data processing hardware 124, enablement anddisablement of the optical transceiver 122 to transmit a downstreamoptical signal 104 d to the optical network unit 140 at a threshold bitrate, resulting in a modulated downstream optical signal 104 d includingencoded channel information.

In some implementations, when the signal indication 128 includes thesignal-received indication 128 b, the method 500 may includeinstructing, by the data processing hardware 124, the opticaltransceiver 122 to continuously transmit the downstream optical signal104 d from the transceiver 122 to the optical network unit 140 accordingto the channel information. The encoded channel information may includean x-bit long delimiter segment and a y-bit long channel segment.

FIG. 6 provides an example arrangement of operations for a method 600 ofestablishing communication between an OLT 120 and an ONU 140 within anoptical access network 105. At block 602, the method includes receiving,at data processing hardware 144, a signal indication 148 from an opticaltransceiver 142 of an optical network unit 140. At block 604, the method600 includes determining, by the data processing hardware 144, whetherthe signal indication 148 includes the loss-of-signal indication 148 a.The signal indication 148 includes a loss-of-signal indication 148 aindicating non-receipt of a downstream optical signal 104 d from anoptical line terminal 120 or a signal-received indication 148 bindicating receipt of the downstream optical signal 104 d from theoptical line terminal 120. At block 606, when the signal indication 148includes the loss-of-signal indication 148 a, the method 600 includesreceiving, at the data processing hardware 144, a modulated downstreamoptical signal 104 d at a threshold bit rate. At block 608, the method600 includes determining, by the data processing hardware 144, encodedchannel information of the modulated downstream optical signal 104 dbased on the threshold bit rate. At block 610, the method 600 includestuning, by the data processing hardware 144, the optical transceiver 142according to the encoded channel information.

In some implementations, the encoded channel information may include anx-bit long delimiter segment and a y-bit long channel segment.Determining the encoded channel information may include rotating thesegments of the encoded channel information, identifying the x-bit longdelimiter segment, and identifying the y-bit long channel segment aschannel information.

FIG. 7 is a schematic view of an example computing device 700 that maybe used to implement the systems and methods described in this document.The computing device 700 is intended to represent various forms ofdigital computers, such as laptops, desktops, workstations, personaldigital assistants, servers, blade servers, mainframes, and otherappropriate computers. The components shown here, their connections andrelationships, and their functions, are meant to be exemplary only, andare not meant to limit implementations of the inventions describedand/or claimed in this document.

The computing device 700 includes a processor 710, memory 720, a storagedevice 730, a high-speed interface/controller 740 connecting to thememory 720 and high-speed expansion ports 750, and a low speedinterface/controller 760 connecting to low speed bus 770 and storagedevice 730. Each of the components 710, 720, 730, 740, 750, and 760, areinterconnected using various busses, and may be mounted on a commonmotherboard or in other manners as appropriate. The processor 710 canprocess instructions for execution within the computing device 700,including instructions stored in the memory 720 or on the storage device730 to display graphical information for a graphical user interface(GUI) on an external input/output device, such as display 780 coupled tohigh speed interface 740. In other implementations, multiple processorsand/or multiple buses may be used, as appropriate, along with multiplememories and types of memory. Also, multiple computing devices 700 maybe connected, with each device providing portions of the necessaryoperations (e.g., as a server bank, a group of blade servers, or amulti-processor system).

The memory 720 stores information non-transitorily within the computingdevice 700. The memory 720 may be a computer-readable medium, a volatilememory unit(s), or non-volatile memory unit(s). The non-transitorymemory 720 may be physical devices used to store programs (e.g.,sequences of instructions) or data (e.g., program state information) ona temporary or permanent basis for use by the computing device 700.Examples of non-volatile memory include, but are not limited to, flashmemory and read-only memory (ROM)/programmable read-only memory(PROM)/erasable programmable read-only memory (EPROM)/electronicallyerasable programmable read-only memory (EEPROM) (e.g., typically usedfor firmware, such as boot programs). Examples of volatile memoryinclude, but are not limited to, random access memory (RAM), dynamicrandom access memory (DRAM), static random access memory (SRAM), phasechange memory (PCM) as well as disks or tapes.

The storage device 730 is capable of providing mass storage for thecomputing device 700. In some implementations, the storage device 730 isa computer-readable medium. In various different implementations, thestorage device 730 may be a floppy disk device, a hard disk device, anoptical disk device, or a tape device, a flash memory or other similarsolid state memory device, or an array of devices, including devices ina storage area network or other configurations. In additionalimplementations, a computer program product is tangibly embodied in aninformation carrier. The computer program product contains instructionsthat, when executed, perform one or more methods, such as thosedescribed above. The information carrier is a computer- ormachine-readable medium, such as the memory 720, the storage device 730,or memory on processor 710.

The high speed controller 740 manages bandwidth-intensive operations forthe computing device 700, while the low speed controller 760 manageslower bandwidth-intensive operations. Such allocation of duties isexemplary only. In some implementations, the high-speed controller 740is coupled to the memory 720, the display 780 (e.g., through a graphicsprocessor or accelerator), and to the high-speed expansion ports 750,which may accept various expansion cards (not shown). In someimplementations, the low-speed controller 760 is coupled to the storagedevice 730 and low-speed expansion port 770. The low-speed expansionport 770, which may include various communication ports (e.g., USB,Bluetooth, Ethernet, wireless Ethernet), may be coupled to one or moreinput/output devices, such as a keyboard, a pointing device, a scanner,or a networking device such as a switch or router, e.g., through anetwork adapter.

The computing device 700 may be implemented in a number of differentforms, as shown in the figure. For example, it may be implemented as astandard server 700 a or multiple times in a group of such servers 700a, as a laptop computer 700 b, or as part of a rack server system 700 c.

Various implementations of the systems and techniques described hereincan be realized in digital electronic and/or optical circuitry,integrated circuitry, specially designed ASICs (application specificintegrated circuits), computer hardware, firmware, software, and/orcombinations thereof. These various implementations can includeimplementation in one or more computer programs that are executableand/or interpretable on a programmable system including at least oneprogrammable processor, which may be special or general purpose, coupledto receive data and instructions from, and to transmit data andinstructions to, a storage system, at least one input device, and atleast one output device.

These computer programs (also known as programs, software, softwareapplications or code) include machine instructions for a programmableprocessor, and can be implemented in a high-level procedural and/orobject-oriented programming language, and/or in assembly/machinelanguage. As used herein, the terms “machine-readable medium” and“computer-readable medium” refer to any computer program product,non-transitory computer readable medium, apparatus and/or device (e.g.,magnetic discs, optical disks, memory, Programmable Logic Devices(PLDs)) used to provide machine instructions and/or data to aprogrammable processor, including a machine-readable medium thatreceives machine instructions as a machine-readable signal. The term“machine-readable signal” refers to any signal used to provide machineinstructions and/or data to a programmable processor.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby special purpose logic circuitry, e.g., an FPGA (field programmablegate array) or an ASIC (application specific integrated circuit).Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices. Computer readable media suitable for storingcomputer program instructions and data include all forms of non-volatilememory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto optical disks; and CD ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, one or more aspects of thedisclosure can be implemented on a computer having a display device,e.g., a CRT (cathode ray tube), LCD (liquid crystal display) monitor, ortouch screen for displaying information to the user and optionally akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. Accordingly, otherimplementations are within the scope of the following claims.

1. A method comprising: receiving, at data processing hardware, a signalindication from an optical transceiver of an optical line terminal, thesignal indication comprising: a loss-of-signal indication indicatingnon-receipt of an upstream optical signal from an optical network unit;or a signal-received indication indicating receipt of the upstreamoptical signal from the optical network unit; determining, by the dataprocessing hardware, whether the signal indication comprises theloss-of-signal indication; when the signal indication comprises theloss-of-signal indication, instructing, by the data processing hardware,the optical transceiver to cease signal transmission from the opticaltransceiver to the optical network unit; and when the signal indicationcomprises the signal-received indication, instructing, by the dataprocessing hardware, the optical transceiver to transmit a downstreamoptical signal from the optical transceiver to the optical network unit.2. The method of claim 1, wherein the optical network unit is configuredto recursively transmit the upstream optical signal at differentwavelengths until receipt of the downstream optical signal.
 3. Themethod of claim 2, wherein the optical network unit is configured torecursively transmit the upstream optical signal at sequentiallydifferent wavelengths within a free spectral range.
 4. The method ofclaim 2, wherein the optical network unit is configured to delaytransmission of a subsequent upstream optical signal after a priorupstream optical signal by a threshold period of time.
 5. The method ofclaim 1, wherein the upstream optical signal and the downstream opticalsignal have different wavelengths.
 6. An optical line terminalcomprising: an optical transceiver; data processing hardware incommunication with the optical transceiver; and memory hardware incommunication with the data processing hardware, the memory hardwarestoring instructions that when executed on the data processing hardwarecause the data processing hardware to perform operations comprising:receiving a signal indication from the optical transceiver, the signalindication comprising: a loss-of-signal indication indicatingnon-receipt of an upstream optical signal from an optical network unit;or a signal-received indication indicating receipt of the upstreamoptical signal from the optical network unit; determining whether thesignal indication comprises the loss-of-signal indication; when thesignal indication comprises the loss-of-signal indication, instructingthe optical transceiver to cease signal transmission from the opticaltransceiver to the optical network unit; and when the signal indicationcomprises the signal-received indication, instructing the opticaltransceiver to transmit a downstream optical signal from the opticaltransceiver to the optical network unit.
 7. The optical line terminal ofclaim 6, wherein the optical network unit is configured to recursivelytransmit the upstream optical signal at different wavelengths untilreceipt of the downstream optical signal.
 8. The optical line terminalof claim 7, wherein the optical network unit is configured to delaytransmission of a subsequent upstream optical signal after a priorupstream optical signal by a threshold period of time.
 9. The opticalline terminal of claim 7, wherein the optical network unit is configuredto recursively transmit the upstream optical signal at sequentiallydifferent wavelengths within a free spectral range.
 10. The optical lineterminal of claim 6, wherein the upstream optical signal and thedownstream optical signal have different wavelengths.
 11. A methodcomprising: instructing, by data processing hardware, an opticaltransceiver to recursively transmit an upstream optical signal atdifferent wavelengths to an optical line terminal; for each transmittedupstream optical signal, determining, by the data processing hardware,whether the optical transceiver received a downstream optical signalfrom the optical line terminal within a threshold period of time; andwhen the optical transceiver received the downstream optical signal fromthe optical line terminal, instructing, by data processing hardware, theoptical transceiver to cease changing wavelengths of the upstreamoptical signal.
 12. The method of claim 11, wherein instructing theoptical transceiver to recursively transmit the upstream optical signalat different wavelengths comprises delaying transmission of a subsequentupstream optical signal after a prior upstream optical signal by athreshold delay.
 13. The method of claim 11, wherein instructing theoptical transceiver to recursively transmit the upstream optical signalat different wavelengths comprises transmitting the upstream opticalsignal at sequentially different wavelengths within a free spectralrange.
 14. The method of claim 11, wherein the upstream optical signaland the downstream optical signal have different wavelengths.
 15. Themethod of claim 11, further comprising storing in memory hardware atleast one of: the wavelength of the upstream optical signal when theoptical transceiver received the downstream optical signal from theoptical line terminal; or the wavelength of the downstream opticalsignal.
 16. An optical network unit comprising: an optical transceiver;data processing hardware in communication with the optical transceiver;and memory hardware in communication with the data processing hardware,the memory hardware storing instructions that when executed on the dataprocessing hardware cause the data processing hardware to performoperations comprising: instructing an optical transceiver to recursivelytransmit an upstream optical signal at different wavelengths to anoptical line terminal; for each transmitted upstream optical signal,determining, whether the optical transceiver received a downstreamoptical signal from the optical line terminal within a threshold periodof time; and when the optical transceiver received the downstreamoptical signal from the optical line terminal, instructing the opticaltransceiver to cease changing wavelengths of the upstream opticalsignal.
 17. The optical network unit of claim 16, wherein instructingthe optical transceiver to recursively transmit the upstream opticalsignal at different wavelengths comprises delaying transmission of asubsequent upstream optical signal after a prior upstream optical signalby a threshold delay.
 18. The optical network unit of claim 16, whereininstructing the optical transceiver to recursively transmit the upstreamoptical signal at different wavelengths comprises transmitting theupstream optical signal at sequentially different wavelengths within afree spectral range.
 19. The optical network unit of claim 16, whereinthe upstream optical signal and the downstream optical signal havedifferent wavelengths.
 20. The optical network unit of claim 16, whereinthe operations further comprise storing in memory hardware incommunication with the data processing hardware at least one of: thewavelength of the upstream optical signal when the optical transceiverreceived the downstream optical signal from the optical line terminal;or the wavelength of the downstream optical signal.
 21. A methodcomprising: receiving, at data processing hardware, a signal indicationfrom an optical transceiver of an optical line terminal, the signalindication comprising: a loss-of-signal indication indicatingnon-receipt of an upstream optical signal from an optical network unit;or a signal-received indication indicating receipt of the upstreamoptical signal from the optical network unit; determining, by the dataprocessing hardware, whether the signal indication comprises theloss-of-signal indication; and when the signal indication comprises theloss-of-signal indication, instructing, by the data processing hardware,enablement and disablement of the optical transceiver to transmit adownstream optical signal to the optical network unit at a threshold bitrate, resulting in a modulated downstream optical signal comprisingencoded channel information.
 22. The method of claim 21, furthercomprising, when the signal indication comprises the signal-receivedindication, instructing, by the data processing hardware, the opticaltransceiver to continuously transmit the downstream optical signal fromthe transceiver to the optical network unit according to the encodedchannel information.
 23. The method of claim 21, wherein the encodedchannel information comprises an x-bit long delimiter segment and ay-bit long channel segment
 24. An optical line terminal comprising: anoptical transceiver; data processing hardware in communication with theoptical transceiver; and memory hardware in communication with the dataprocessing hardware, the memory hardware storing instructions that whenexecuted on the data processing hardware cause the data processinghardware to perform operations comprising: receiving a signal indicationfrom the optical transceiver, the signal indication comprising: aloss-of-signal indication indicating non-receipt of an upstream opticalsignal from an optical network unit; or a signal-received indicationindicating receipt of the upstream optical signal from the opticalnetwork unit; determining whether the signal indication comprises theloss-of-signal indication; and when the signal indication comprises theloss-of-signal indication, instructing, by the data processing hardware,enablement and disablement of the optical transceiver to transmit adownstream optical signal to the optical network unit at a threshold bitrate, resulting in a modulated downstream optical signal comprisingencoded channel information.
 25. The optical line terminal of claim 24,wherein the operations further comprise, when the signal indicationcomprises the signal-received indication, instructing the opticaltransceiver to continuously transmit the downstream optical signal fromthe transceiver to the optical network unit according to the encodedchannel information.
 26. The optical line terminal of claim 24, whereinthe encoded channel information comprises an x-bit long delimitersegment and a y-bit long channel segment
 27. A method comprising:receiving, at data processing hardware, a signal indication from anoptical transceiver of an optical network unit, the signal indicationcomprising: a loss-of-signal indication indicating non-receipt of adownstream optical signal from an optical line terminal; or asignal-received indication indicating receipt of the downstream opticalsignal from the optical line terminal; determining, by the dataprocessing hardware, whether the signal indication comprises theloss-of-signal indication; and when the signal indication comprises theloss-of-signal indication: receiving, at the data processing hardware, amodulated downstream optical signal at a threshold bit rate;determining, by the data processing hardware, encoded channelinformation of the modulated downstream optical signal based on thethreshold bit rate; and tuning, by the data processing hardware, theoptical transceiver according the encoded channel information.
 28. Themethod of claim 27, wherein the encoded channel information comprises anx-bit long delimiter segment and a y-bit long channel segment.
 29. Themethod of claim 28, wherein determining the encoded channel informationcomprises: rotating the segments of the encoded channel information;identifying the x-bit long delimiter segment; and identifying the y-bitlong channel segment as channel information.
 30. An optical network unitcomprising: an optical transceiver; data processing hardware incommunication with the optical transceiver; and memory hardware incommunication with the data processing hardware, the memory hardwarestoring instructions that when executed on the data processing hardwarecause the data processing hardware to perform operations comprising:receiving a signal indication from the optical transceiver, the signalindication comprising: a loss-of-signal indication indicatingnon-receipt of a downstream optical signal from an optical lineterminal; or a signal-received indication indicating receipt of thedownstream optical signal from the optical line terminal; determiningwhether the signal indication comprises the loss-of-signal indication;and when the signal indication comprises the loss-of-signal indication:receiving a modulated downstream optical signal at a threshold bit rate;determining encoded channel information of the modulated downstreamoptical signal based on the threshold bit rate; and tuning the opticaltransceiver according to the encoded channel information.
 31. Theoptical network unit of claim 30, wherein the encoded channelinformation comprises an x-bit long delimiter segment and a y-bit longchannel segment.
 32. The optical network unit of claim 31, whereindetermining the encoded channel information comprises: rotating thesegments of the encoded channel information; identifying the x-bit longdelimiter segment; and identifying the y-bit long channel segment aschannel information.